This invention relates to a control device of a vehicle mounted, charging generator driven by, for example, an internal combustion engine thereof for regulating an output voltage of the motor generator to a predetermined value and, particularly, to a protection circuit for use in a such control device for protecting an output transistor of the control device when excess current flows due to, for example, a short-circuit of a field winding of the generator.
FIG. 1 shows an example of a conventional voltage regulator of a vehicle-mounted generator in which a three-phase a.c. generator 1 includes a three-phase star wired armature winding 101 and a field winding 102. A full wave rectifier 2 for rectifying an a.c. output of the generator 1 includes a first d.c. output terminal 201, a second d.c. output termnal 202 and a grounding terminal 203. A voltage regulator 3 controls an output voltage of the generator 1 to be a first predetermined value by controlling a field current flowing through the field winding 102 and includes a surge absorbing diode 301 connected across the field winding 102, output transistors 302 and 303 for on-off controlling the field current, a resistor 304 constituting base circuits of the transistors 302 and 303, a transistor 305 for on-off controlling the output transistors 302 and 303, a Zener diode 306, responsive to the output voltage of the generator 1, which turns on when the output voltage becomes equal to or higher than the first predetermined voltage, resistors 307 and 308 connected in series with each other and constituting a voltage divider circuit for the output voltage of the generator 1, a limiting transistor 309 connected in parallel to the control transistor 305 for on-off controlling the transistors 302 and 303, level shift diodes 310 and 311 connected in series with a base terminal of the transistor 309, a first detection diode 312 for detecting a collector voltage of the control transistor 305, a second detection doide 313 for detecting a collector voltage of the output transistors 302 and 303, a base resistor 314 disposed in the base circuit of the limiting transistor 309, a battery 4, a key switch 5 and a charge indicator lamp 6.
In operation, when there is no short-circuit between phases of the field winding 102 and the key switch 5 is closed to start the engine, a base current flows from the battery 4 through the key swtich 5, the charge indicator lamp 6 and the base resistor 304 of the output transistor 303 to turn the latter transistor on and thus the output transistor 302 is turned on. Upon the conduction of the transistor 302, a field current flows from the battery 4 through the key switch 5, the charge indicator lamp 6, the field winding 102 and the transistor 302 and thus a field magnetomotive force is generated. At this time, a collector-emitter voltage of the output transistor 302 is lower than a sum base-emitter voltage of the limiting transistor 309 and a forward voltage drop of level shift diodes 310 and 311 and thus the transistor 309 is turned off.
Then when the engine is started to drive generator 1 and an output voltage of the generator 1 exceeds the first predetermined value of the voltage regulator 3, the Zener diode 306 and the control transistor 305 are turned-on and the output transistors 302 and 303 are turned-off. Therefore, the output voltage of the generator 1 is lowered. Since, at this time, the control transistor 305 is conductive, a collector-emitter voltage of the latter becomes lower than a sum of the base-emitter voltage of the control transistor 309 and the forward voltage drop of the diodes 310 and 311 and thus the limiting transistor 309 is kept non-conductive.
When the output voltage of the generator is lowered to the first predetermined level, the output transistors 302 and 303 are turned on again to energize the field winding 102 to thereby increase the output voltage of the generator. The output voltage of the generator 1 can be regulated at the first predetermined level upon repetitions of the above mentioned operations.
On the other hand, if the field winding 102 is short-circuited or there is an inter-phase short-circuit, when the key switch 5 is closed while the engine is in the rest state, the collector current of the output transistor 302 is sharply increased. However, since the base current of the transistor 303 is limited by the resistor 304, amplification is not sufficient and the voltage between the collector and the emitter of the transistor 302 becomes equal to or above a sum of the base-emitter voltage of the limiting transistor 309 and the forward voltage drop of the diodes 310 and 311. Since the engine is in the rest state at this time and there is no output generated by the generator 1, the Zener diode 306 and the control transistor 305 are non-conductive. Therefore, a current flows through the resistor 314, the diodes 310 and 311 and the base-emitter of the limiting transistor 309 to turn the latter on. Thus, the output transistors 302 and 303 are cut off, so that a breakdown of the output transistor 302 and 303 due to very large collector currents can be prevented.
In the conventional device shown in FIG. 1 or 2, the collector current of the output transistor 302 and hence the collector current of the output transistor 303 necessary to turn-on the limiting transistor 309 is usually set at a very large value compared with a collector current in a normal conductor state. Therefore, there is no large increase of current in a case of inter-phase short-circuit of the field winding 102. Further, in a conventional system where the field current is supplied from an output terminal of a second rectifier, the output voltage of the generator 1 is lowered in a case of rare short-circuit. Therefore, there is no large increase of the collector currents of the transistors 302 and 303 and the collector potentials of the transistors 302 and 303 do not increase to values at which the limiting transistors 309 is turned-on. Therefore, the transistor 302 may break down.
FIG. 2 shows another example of the conventional device in which a resistor 300 is connected in parallel to the charge indicator lamp 6 so that even if the lamp 6 is broken it impossible to supply the initial energizing current to the generator 1 and the limiting transistor 309, the level shift diodes 310 and 311, the detector diodes 312 and 313 and the resistor 314 in FIG. 1 are omitted.
The problem inherent in the conventional device in FIG. 1 also exists in the example shown in FIG. 2. That is, in FIG. 2, when there is an inter-phase short-circuit when the field winding 102 is in operation or when the failed voltage regulator 3 is replaced by a new voltage regulator while such inter-phase short-circuit has not been indicated, a very large current which is larger than a predetermined field current may flow through the output transistor 302 causing the latter to break down.